FIG. 1 is functional diagram illustrating an example of a prior system 100 that includes three resistive combiners 108, 110, and 112 and two directional couplers 114 and 116. In the example, the system 100 includes a first signal generator 102 that provides a differential data signal to the directional couplers 114 and 116. The system 100 also includes a second signal generator 104 that provides an input signal, such as a 2.1 GHz sine wave, to a balun 105. Balun 105 creates a differential mode signal from the single-ended signal it receives from signal generator 104, and provides one end of the differential mode signal to resistive combiner 108 and the other end to resistive combiner 110. The system 100 also includes a common mode (CM) signal generator 106 that provides a CM signal to an amplifier 107 before being passed to resistive combiner 112.
Resistive combiner 112 is electrically coupled between the amplifier 107 and each of the other two resistive combiners 108 and 110, which also receive signals from the balun 105. In this manner the resistive combiners 108, 110, and 112 effectively combine the input signal from the second signal generator 104 with the CM signal from the CM signal generator 106. Directional couplers 114 and 116 effectively apply the differential data from the first signal generator 102 to the resulting signals from resistive combiners 108 and 110, respectively.
Prior implementations such as the one illustrated by FIG. 1 are physically large and expensive. Such implementations also have a number of disadvantages. For example, loss from input data to output data is generally higher at high frequencies than at low frequencies. In other words, such loss is not flat aver frequency. Also, loss from other inputs to output data tends to roll off as frequency decreases and can be inadequate at certain frequencies, some of which can be important.
FIG. 2 is a functional diagram illustrating an example of a prior apparatus 200 that includes five resistive combiners 202 and 206-212, balun 204, and no directional couplers. In the example, resistive combiner 202 may receive the common mode (CM) input and balun 204 may receive the differential mode (DM) input. Resistive combiners 206 and 208 are each electrically coupled with resistive combiner 202 and balun 204 and also with resistive combiners 210 and 212, respectively. Resistive combiners 210 and 212 are typically pickoff tees.
Prior systems such as would use the apparatus of FIG. 2 generally use resistive combining, e.g., pickoff tees 210 and 212, for a flatter loss in input data to output data aver frequency. Such systems have loss in the input data to output data path that is relatively low (2 dB). However, a consequence is that a pickoff tee that injects the differential mode and common mode signals onto the data generally has higher impedance at the injection point than the system impedance of 50 Ohms, thereby causing reflections that lead to a non-flat transfer function from the CM input and DM input to the data output.
Another disadvantage of prior approaches is that, in order to avoid further impedance mismatches, the balun that the DM input signal goes into, e.g., balun 204 of FIG. 2, must have a 2:1 turn ratio, which limits its frequency response relative to what is achievable with transmission line baluns having a 1:1 ratio.
FIG. 3 is a functional diagram illustrating an example of a prior system 300 that includes four resistive combiners 310-316 that effectively combine a DM signal from a DM source 302 and CM signal from two separate CM sources 306 and 308 with an input signal from a pattern generator 304. As with the apparatus 200 of FIG. 2, this system 300 also uses resistive combining for flat loss in the input data to output data over frequency. The system 300 also has the disadvantage of high through loss, i.e., 6 dB, from the input data to the output data. It also cannot receive as input a single-ended differential mode input but instead requires two signals that are 180 degrees out of phase and each have 50 Ohm impedance.
FIG. 4 is a graphing that illustrates an example 400 of a data signal resulting from a prior system or apparatus such as that illustrated by FIG. 1. One having ordinary skill in the art will appreciate that the attenuation over frequency in the example 400 is not flat, thus resulting in a data signal that is not clean.
Accordingly, a need remains for improved systems that add CM and DM signals onto differential data.